This invention relates, in general, to a method of and apparatus for selectively changing the sensitivity of a photometer to different portions of a scene, as well as indicating which sensitivity preference is being used.
Automatic exposure control systems are well-known in the photographic arts. Typically, such systems include a photometer having a photoresponsive transducer and a lens element or system. The lens system receives scene radiation from a preselected area of the scene to be photographed and directs it onto the transducer's photosensitive surface. The corresponding transducer output signal may be utilized to predict a photographic exposure condition or actually control the closing of an automatic shutter mechanism at an appropriate film exposure condition. Most of the exposure control systems used in the prior art employ a single photocell. Use of a single cell photometer, however, presents certain drawbacks. This is because they react to the average scene brightness of a field to be photographed. Unfortunately, the average scene or field brightness usually differs from the brightness of selected zones of photographic interest. For instance, central or lower zones of the field have been found to be more representative of the brightness of the subject. Thus, when the brightness of the peripheral or upper field zone contrasts with the brightness of central or lower field zones, an incorrect exposure for the more important subject normally occupying the center or lower zones results. Consider, for example, a backlit scene where the illumination level of the subject of principal interest is much lower than the illumination level of the background. One common picture taking situation which exhibits backlighting is when the sun is in the background behind the subject. In these instances, such automatic exposure control systems set exposure for the higher background illumination. As a result, the subject of principal interest is underexposed.
Further, consider a forelit scene. This may occur where the subject is brightly illuminated by sunlight, or an artificial light source, while the areas around the subject exhibit subdued illumination because of shadows or a dark colored light absorbing background. Obviously, difficulties arise if the photometer is configured to take an average brightness reading.
To correct for the noted drawbacks several proposals have been put forward. One has been to alter the aiming of the photometer lens system so that it evaluates scene radiation from different scene zones or portions. This is typically done in photographic systems having distinct exposure modes, for example, systems having distinct flash and ambient modes. Examples of such photocell aiming altering devices are shown in the following U.S. Pat. Nos. 3,511,142; 3,529,523 and 4,173,400. These systems are also limited because such changes are responsive only to the presence or absence of a flash device.
Another proposal is to use a multi-photocell array. U.S. Pat. Nos.: 3,511,142; 3,529,523 and 4,188,104; 4,214,826; 4,285,583; 4,285,584; 4,306,787; 4,309,090 and 4,309,091 disclose photometric devices using an array of photocells arranged so that under certain conditions, each photocell receives light from a preselected portion of the entire scene. One limitation of some of these systems besides cost is that they compare the light level reading of the center portion of the scene with readings from the sides and top and bottom of the scene based on the assumption that the subject of principal interest will be centered in the viewfinder. In many instances the assumption will be valid. However, many times it will not be valid in that the subject of principal interest may well be located about the periphery of the scene boundary. In such instances an erroneous conclusion will be reached by the logic as to the classification of scene lighting conditions and more likely than not the subject of principal interest will be either over or underexposed.
In U.S. Pat. No. 3,406,294 there is disclosed a light exposure meter having means to adjust the acceptance angle of the scene radiation impinging upon a photocell. The system is rather complicated because it includes a plurality of light tubes through which light passes before impinging on the photocell. The angular orientation of these tubes is adjustable by moving a sleeve in a direction parallel to the axis of the system. In this manner, the outer ends of the tubes are moved relative to each other. Besides being relatively complicated, the system only changes the angle of light acceptance and not the directional response characteristics of the photometer.
U.S. Pat. No. 3,000,281 discloses an automatic setting device for a photographic camera in which a rotatable disc having a plurality of different size apertures therein may be rotated so that selective ones of the apertures can control the amount of scene radiation striking the photocell.
None of the known prior art can control exposure by allowing the operator a great degree of versatility in changing the sensitivity including the angular directional response in a reliable and compact manner let alone automatically in response to scene lighting conditions. Additionally, none make provision for the foregoing while also making provision for conveniently indicating the altered directional response characteristics of the photometer.